Ntareme, Hervé

Bagula, Antoine

University of Cape Town.

2008 (English)In: Proceedings of the International Conference on Mobile Technology, Applications, and Systems, 2008Conference paper, Published paper (Refereed)

Abstract [en]

This paper revisits the link quality issue in Wireless Sensor Networks (WSN) by studying the temporal and energy characteristics of a 2, 4Ghz sensor network in an outdoor environment. Using different values of output power and sampling period, we analyze battery behavior in motes placed at different distances and show that farther motes have a shorter battery life. Our experimental results suggest that when deployed in real world deployments, the sampling periods of sensor networks be adjusted according to distance to normalize battery lifetime and a more accurate energy-aware routing protocol be developed.

Abstract [en]

Wireless Sensor Networks (WSNs) provide a way to bridge the gap between the physical and the virtual worlds. They promise unprecedented abilities to observe and understand large-scale, real-world phenomena at a fine spatial-temporal resolution. Their application in Developing Countries is evenmore interesting: they can help solve problems that affect communities. The number of potential applications in such an environment is huge: water monitoringand crop modeling are just two examples.

This thesis will analyze the potential of WSNs in Developing Countries, and tackle three of the main problems that their deployment poses:

1. Power. Power consumption is an important issue in deployments. For WSN nodes this is a well-addressed issue. Most commercial solutions today assume that WSN gateways (the devices that provide the interface between the nodes and the network infrastructure) will encounter ideal scenarios in terms of power when deployed. In a Developing World scenario, the gateway must operate with bounded energy supplies. The gateway should have sufficient stored power to save sensed data with high probabilities of service interruptions due to power loss.

2. Connectivity. Network connectivity in many Developing Countries is unreliable. Data gathered by the sensor nodes (motes) must therefore be stored safely in the gateway and transferred when a network connection is available. WSN deployments can encounter different network topologies such as wired, wireless and mesh, and should be flexible enough to interact with each of them. A two layer architecture with long wireless links above a wireless sensor network is a practical alternative for providing connectivity in the access network.

3. Quality of WSN links. To implement reliable and robust sensor networks, we need to understand the variation of link quality and battery behavior in a real world environment. Low-power transmitters have a limited range, and it is important to understand communication patterns. Energy is the scarcest resource of WSN motes, and it determines the lifetime of WSNs. Motes are meant to be deployed in various environments, including remote and hostile regions; consequently, they must use little power and one need to make sure that all batteries last the same amount of time. Also, battery level has an impact on routing.

There is still research to be carried out to make WSNs suitable for deploymen tin Developing Regions. Following the most recent developments of sensor networks, this thesis discusses what ICT4D researchers could do to accelerate the dissemination of this new technology and proposes prototype solutions for some of the three problems mentioned above.

These include:

• Efficient link quality models. Link quality models are important tools upon which the deployment of wireless sensor networks depends. They allow the selection of efficient working parameters that enable the information collected by sensor networks to be routed efficiently from WSN nodes to a sensor base station. Using two testbed WSNs based on two different technologies, we analyzed spatial and temporal behavior of link quality and derived good working parameters for sensor network deployments. These parameters have been used in related works to design new routing protocols for sensor networks.

• Robust and flexible gateways. Building upon the assumption that a WSN gateway to be deployed in developing regions should be designed at low cost with battery backup to maintain a continuous supply of electric power in absence of power grid, we proposed in this thesis two low-cost solutions that 1) meet low-power consumption and high storage capabilities constraints 2) are based on web technologies and (3) allow long range deployment for information dissemination. These solutions are developed around two smart board systems.

• Long wireless links. Besides power energy limitations, limited range is one of the main factors which has delayed large scale deployments of wireless sensor networks. Building upon the assumption that next generation sensor networks will be employed in multilayer network environments with a WiFi gateway network layered above islands of sensor networks, we presented the deployment of several long wireless links. These prototypes revealed that long distance links can be a practical, inexpensive alternative for connecting wireless sensor networks while providing access to Internet in Developing Countries.

• Water quality management. We developed a water quality monitoring system to de deployed in Malawi. Water quality measurementadds another dimension to the issue of power consumption of the WSN system since one has to take into account the contribution of the energy consumed by the water quality sensors in the overall energy consumption of the water monitoring system. As low-power sensors for water quality are not yet commercially available, we proposed an energy consumption minimization strategy where a wake-up mechanism that triggers sleeping/wake-up modes is used to reduce energy consumption.